The distribution of radionuclides inside the body in 3 dimensions provides unique and valuable information for studying biochemical function. A new detection system optimized for obtaining computerized transaxial reconstructions of the activity distribution has been designed. This system can provide a hundredfold gain in sensitivity over a conventional camera by using electronic instead of mechanical collimation for single gamma emitting radionuclides. Collimation is obtained by requiring a sequential interaction of the emitted gamma rays with two detector systems, both of which are position and energy sensitive. The first detector is thin, and optimized for obtaining Compton interactions, while the second detector is thick, and geometricaaly arranged behind the first detector to capture as large a fraction as possible of the scattered gamma rays. By recording events in coincidence between the two detectors for a selected amount of energy deposited in the first detector, and the remaining amount deposited in the second, the incident angle of the primary gamma ray on the first detector can be deduced. The activity is therefore localized on the surface of the cone with its apex on the first detector. Given enough such surface integrals for arrays of detectors situated around the patient, it is possible to reconstruct the radionuclide density at each point in the viewed volume. The spatial resolution depends mainly on the energy resolution of the thin detectors. Multiple transverse layers of the object can be viewed from many directions simultaneously, and computerized transaxial reconstructions can be obtained for multiple layers without requiring any mechanical motion.